In order to study the residues of phosphorous insecticides in Mazandaran district of Iran, Ethion, Azinphosmethyl, Diazinon and Malathion four most used pesticides were chosen and river water samples were collected in April to Sept. 2000, throughout the Mazandaran state. Thin layer chromatography was used, since it was the best applicable method in area with reasonably high level of sensitivity up to 0-01 ppm and acceptable recovery of 80percent. Four hundred eighty samples were collected and tested for. The tests indicated maximum amount of residues found in river water was 2.7 ppm for Ethion, and sill detectable up to 0.2 ppm after 4 months at station No.2 of Torkrood. The highest level belonged to the day after application. Three ppm of Azinphosmethyl were observed after first day of application and 0.2 ppm station 2 of Tajan after 5 months 3.1 ppm Malathion and 0.2 ppm in station. No.2 of Tajan river and 3.8 ppm of Diazinon at first day and 0.1 ppm up to five months only in the station No.2 of Tajan river. The duration of showing positive test were 4, 4, 5 and 3 months for Ethion, Azinphosmethyl, Diazinon and Malathion respectively. Matching test results with the environmental condition, proved strong effect of environmental factors, such as temperature, pH of water, amount of rain, physical and chemical characteristics of pesticide, and the time and method of application.